1. Field of the Invention
The present invention relates to a liquid level sensor for an automatic chemical analyzer, and more particularly to a capacitive liquid level sensor which is used not only as a liquid level sensor, but also as a pipetting tube in the automatic chemical analyzer.
2. Discussion of the Background
An automatic chemical analyzer automatically analyzes a plurality of samples such as patient serums according to a number of analysis items. The samples in a sample container are dispensed to an array of reaction vessels using a pipetting tube. Reagent solutions selected in accordance with the analysis items are fed to the reaction vessels using another pipetting tube. The reaction solutions in the reaction vessels are analyzed, for example, by ion selecting electrodes for measuring ion activities like sodium, potassium and chlorine or by a spectrometer for spectral analysis. The results of such analyses are displayed on a monitor or typed, analysis item by item, and, patient by patient.
In the conventional automatic chemical analyzer, a liquid level sensor cooperates with such a pipetting tube for detecting the liquid level of the samples in the sample container or reagent solutions of the reagent solution tanks. A deep immersion of the pipetting tube results in excess liquid sticking to outside of the pipetting tube. This excess liquid, not only decreases accuracy of the pipetting, but also, causes contamination with another sample or reagent solution. The inaccuracy of the pipetting and contamination caused by the excess liquid sticking to the outside of the pipetting tube makes the analysis results inaccurate and less reliable.
In other words, shallow immersion of the pipetting tube gives accurate and reliable analysis results. A liquid level sensor enables such a shallow immersion of the pipetting tube and avoids deep and blind immersion of it.
A conventional liquid level sensor is described in for example, U.S. Pat. No. 4,451,433. The conventional liquid level sensor comprises a pair of electrically conductive members. One is a pipetting tube made of a chemical proof metal such as platinum or stainless steel and the other is an electrode made of a chemical proof metal wire. Such a pair of electrically conductive members provides a detection signal when both are immersed into a reagent solution or samples.
However such a conventional liquid level sensor may cause inaccurate and contaminated pipetting because the electrode accompanying the pipetting tube is immersed as well as the pipetting tube. Liquid sticking to the electrode is liable to produce less accurate and more contaminated pipetting.
Further, it is difficult in the conventional liquid level sensor to watch and evaluate whether or not the suction of the pipetting tube is normal.